Ammonium thiosulfate, (hereafter "ATS"), is typically prepared on a commercial scale by gas injection of sulfur dioxide and ammonia into an aqueous solution. See, for example, GB Patent No. 1287326; and U.S. Pat. Nos. 2,315,534; 2,412,607; 3,431,070; 3,473,891; 3,493,337; 3,524,724; 3,937,793; and 4,478,807. The resulting aqueous solution of ATS has found considerable use as a photographic fixative or a component thereof.
One important requirement for photographic fixers is that the ATS be sulfur-free because sulfur is insoluble, and any appreciable sulfur concentration would create a turbid solution and leave an objectionable residue on the photographic film and paper drying processes. ATS, further, tends to decompose into sulfur (and other compounds) upon exposure to heat and humidity. Thus, such decomposition needs to be avoided if the ATS-containing material is to be used in a photographic fixer environment.
Recently, the art has sought to provide dry, e.g., powder, formulations of photographic fixers in convenient packaging sizes, which formulations contain solid crystalline ATS. The source of the crystalline ATS is derived by precipitation from conventional aqueous solutions containing ATS. Such precipitation techniques suffer from a number of disadvantages including being slow, laborious, costly, and incurring problems in recycling or disposing of mother liquor remaining from the crystallization process.
Moreover, because ATS decomposition into, e.g., sulfur increases significantly at elevated temperatures, typical processes for accelerating the recovery of ATS crystalline solid, e.g., by the application of various drying processes at elevated temperatures, are at best problematic and heretofore have not achieved satisfactory results, particularly on a commercially acceptable scale.
Several approaches have been tried to mitigate ATS decomposition in these preparation techniques while optimizing the production of the dry solid. None, however, has proven entirely satisfactory.
Metzger, U.S. Pat. No. 3,937,793, discloses a complex process of reacting ammonia, oxygen, and sulfur dioxide in an ATS solution to form ammonium bisulfite and ammonium sulfite and then converting the intermediate bisulfite and sulfite to ATS by heating followed by crystallization.
Ingraham, U.S. Pat. No. 3,345,131, discloses that solid ATS can be stabilized by high pressure compression into briquettes. However, the product of this process would not dissolve as rapidly as would be required of a powder fixative mixture.
Brinkman, U.S. Pat. No. 4,378,339, discloses a somewhat moist ATS that appears to be more stable than anhydrous ATS but would not meet the flowability criteria of today's marketplace.
Ziegler, U.S. Pat. No. 3,350,168, discloses that the decomposition of anhydrous, crystalline ATS can be prevented by the addition of 0.01 to 10% of a second ammonium salt sufficiently volatile or unstable to provide an ammonia atmosphere surrounding the ATS. This patent further discloses that heating the thus preserved ATS to 220.degree. F. in a closed bottle did not cause any noticeable decomposition.
Mack, U.S. Pat. No. 3,512,929, discloses that sodium thiosulfate when admixed with crystalline ATS has a stabilizing effect presumably by removing the last traces of water from the ATS.
GB 1310242 to Hoechst discloses a method for stabilizing crystalline ATS and ATS aqueous solutions using 0.1 to 8.0% (w/w) additions of sodium sulfite and 0.5 to 3.5% (w/w) additions of dicyanodiamide.
Although these aforementioned attempts at stabilizing ATS solid have met with varying degrees of success, no commercially viable means of producing dry, freely flowable, sulfur-free ATS powder has been accomplished.
Another technique for manufacturing a solid photographic fixer is the granulation technique described by Kuhnert in U.S. Pat. No. 5,055,384. The Kuhnert patent discloses a process for preparing a flowable ATS-based fixer granulate comprising milling all of the fixer powder ingredients, including precipitated, crystalline ATS; then introducing the solid blend into a fluidized bed, contacting the fluidized bed with water, heating the bed to 60.degree. C. for a short period under atmospheric pressure, followed by vacuum drying at room temperature. However, as is apparent, the process employs precipitated ATS whose disadvantages were discussed earlier.
Thus, the need still exists for a process for producing a dry product containing ATS which is not subject to the foregoing disadvantages and in particular the problems associated with decomposition.